This invention relates to x-ray diffraction apparatus and method and more particularly to an x-ray diffraction apparatus provided with means for correcting data in qualitative analyses and a method of x-ray diffraction analysis by making use of such an apparatus.
When a beam of x-rays is made incident on a crystalline surface, the beams reflected by the latticed crystalline surface interfere and the intensity of the diffracted beams in directions satisfying Bragg""s formula is increased while the beams diffracted at other angles are cancelled and are not observed. The x-ray diffraction method makes use of this phenomenon and carries out qualitative analysis of a sample material from the relative intensity of diffracted x-ray beams corresponding to the lattice surface spacing which is characteristic of the material. X-ray diffraction apparatus using a goniometer have been used for measuring the angle between an x-ray beam incident on a sample and a beam diffracted thereby and its intensity.
Qualitative analyses by x-ray diffraction are carried out by comparing the intensity pattern of diffracted x-rays obtained by a measurement on a sample with reference patterns of diffracted x-rays obtained by measurements on standard (or reference) materials. Such reference patterns of standard materials are usually used in a preliminarily prepared form such as a database.
Both a slit with a fixed aperture (xe2x80x9ca fixed slitxe2x80x9d) and a slit with a variable aperture (xe2x80x9ca variable slitxe2x80x9d) are used to serve as a divergence slit for determining the width of the x-rays to be made incident on a sample surface. If a fixed slit is used, the width of the irradiated area on the sample (xe2x80x9cthe irradiated widthxe2x80x9d) varies as the angle of diffraction is changed. If a variable slit is used, on the other hand, the angle of the opening (xe2x80x9cthe divergence anglexe2x80x9d or xe2x80x9cthe angle of divergencexe2x80x9d) is caused to change with the diffraction angle such that the irradiated width will remain constant. FIG. 7A is a graph which shows the relationship between the angle of diffraction (2xcex8) and the irradiated width (2A) when a fixed slit is used, and FIG. 7B is a graph which shows the relationship between the angle of diffraction (2xcex8) and the irradiated width (2A) when a variable slit is used to keep the irradiated width constant (shown by a broken line).
If a fixed slit is used, the angle of divergence xcex2 is a fixed angle corresponding to the selected slit, independent of the angle of diffraction, and the irradiated width changes according to the angle of diffraction. If the broken line in FIG. 7A is taken as representing an effective width of a sample, it can be understood that only a portion of the effective width is irradiated at a large angle of diffraction but that the irradiated width becomes greater than the effective width at a small angle of diffraction, a portion of the incident x-ray beam becoming wasted. If a variable slit is used, on the other hand, the angle of divergence xcex2 is controlled as the angle of diffraction is varied such that the irradiated width will stay constant independent of the angle of diffraction (as shown by the broken line in FIG. 7B).
Qualitative analyses are carried out by comparing measured intensity data of diffracted x-rays with reference intensity data and based upon the peak positions and the ratios among the peak intensities. Ordinarily, the intensity data of diffracted x-rays of standard materials are obtained by using a fixed divergence slit. For obtaining intensity data of diffracted x-rays from a target sample, however, use may be made of a variable slit as well as a fixed slit. When measured data obtained by using a variable slit are compared with reference data obtained by using a fixed slit, or when measured data obtained by using a fixed slit are compared with reference data obtained by using a variable slit, the comparison of intensity ratio of diffracted x-rays cannot be made under the same conditions because the irradiated width is constant with a variable slit but varies according to the angle of diffraction with a fixed slit. Even if reference data and measured data are each obtained by using a fixed slit, furthermore, the width of the irradiated area may be different if the fixed slits have different angles of divergence. In such a situation, the intensity of the diffracted x-rays will become different even at a same angle of diffraction and the intensity ratio of diffracted x-rays cannot be compared under the same conditions.
In summary, conventional x-ray diffraction apparatus could not carry out accurate qualitative analyses because of the error which will result between measured and reference intensity data when the measured and reference data are obtained under different conditions and, in particular, if different divergence slits are used.
It is therefore an object of this invention to provide an x-ray diffraction apparatus and method capable of comparing measured data and reference data accurately for an accurate qualitative analysis even where the measured and reference data are obtained under different conditions and, in particular, by using different divergence slits.
When measured and reference data are compared in a qualitative analysis by x-ray diffraction according to this invention, the measured data and/or the reference data are corrected so as to make the conditions of measurement of these data equal such that the ratio of measured and reference intensity data can be compared accurately and the qualitative analysis can be carried out accurately. The invention is particularly addressed to the correction of the errors in the intensity ratios caused by using different divergence slits.
An x-ray diffraction apparatus embodying this invention may be characterized as including a data correcting means for making corrections on the measured and/or reference intensity data and thereby correcting the intensity ratio according to the width of the divergence slit. The divergence slit is a slit for determining the divergence angle of the x-rays and hence the area on the sample irradiated by the x-rays and may be a fixed slit or a variable slit.
Aforementioned measured and reference data are intensity data of diffracted x-rays obtained respectively from a target sample and reference samples. The difference in the choice of whether to use a fixed slit or a variable slit for making the width of the irradiated area constant appears only as the difference in the width of the irradiated area. The data correcting means is for correcting the difference between the intensity levels of diffracted x-rays caused by the difference in the width of irradiated area, depending on the kind of divergence slit. Either or both of the measured and reference data are corrected by using a single selected width as a standard width. By thus making a correction or corrections, it becomes possible to compare the intensities of diffracted x-rays of measured and standard data under the same conditions. The data correcting means serves to use the ratio between the width of the area on a sample irradiated by x-rays when a fixed slit is used and that of a fixed irradiated area when a variable slit is used and to multiply either the measured data or the reference data with this ratio to thereby match the intensity of either to the intensity level of the other. It may also use a selected standard width different from either of the widths corresponding to the measured and reference data and multiply correspondingly the ratios between these widths of the irradiated areas and this selected standard width to the measured and reference data so as to match their intensity levels at this selected standard width.